In the last 15 years or so the speed of travel for tissue webs has not increased to any great extent and earlier developments as regards speed have evened out. This is considered to be a result of increasing demands for high-quality tissue and also of difficulties in maintaining the higher production speeds attained in other parts of the paper machine in its dry end also. These difficulties are caused, for instance, by increased vibration in the web at increasing speeds, and problems with web rupture encountered when threading while changing reels since tissue paper has low basis weight and low tensile strength.
Threading means that a web end is pulled along through a paper or cardboard machine by a leader consisting of a strip of the web which may initially be e.g., 400-500 mm wide but gradually becomes wider until it extends across the entire width of the web. The leader is cut out in the continuous web, starting either at one edge or somewhere at an optional distance from either edge of the web, whereas its length is determined by the time it takes for the tip to extend across the entire width of the web. Due to the high web speed the leader may be very long, 180-200 m, and this incurs considerable costs for the paper mill since the cut part of the paper web must be discarded for each paper reel. Threading can be performed with cords, compressed air or manually in machines running at low speed. Currently the safest and most efficient automatic threading means consists of a blower box which, with the aid of compressed air carries the leader to the nip between the new reeling drum and the surface winding drum.
Adhesive is often used, mixed with water to ensure that the leader adheres to a new reel core, or to the paper reel already started if a web rupture has occurred during reeling. The adhesive is applied either manually, using a large brush or spray gun, or automatically with the aid of glue nozzles, generally placed at one side of the paper web close to the primary arms. Serious safety aspects naturally arise as regards this manual application. Unsuccessful reel switching and the need to clean up after glue has been spilled during application, have a significant influence on the total capacity of the machine, particularly since the spilled glue forms lumps with the paper dust falling down as a result of machine vibrations and may thus constitute one of the reasons for web ruptures.
Sanitary tissue products, usually manufactured of tissue paper, are extremely market-sensitive and the quality of the product is therefore often more important than its quantity. It is thus important that during reeling the paper reel acquires several important properties, i.e., homogeneity and lack of wrinkles, tears or folds. Furthermore, high efficiency in the following conversion machines can only be achieved if the reels of paper from the paper machine have a homogenous high quality.
Soft paper with low strength must be reeled carefully in order to keep the paper qualities such as density and elasticity as constant as possible throughout the reel. The two main parameters affecting reel density are web tension and radial pressure at the nip of the reel-up. Lower nip pressure is important in order to obtain lower average density.
The thickness and elasticity decrease from the outside of the reel radially towards its center. This is because the compressive stress built up in the paper reel during reeling compresses the radially inner parts of the reel, thereby causing a loss of thickness in the inner web layers. This effect increases if the reel is stored for too long before being rewound or converted.
Reeling problems arise when a new reel of paper is commenced with the aid of the tapered leader as mentioned above, since the web turns applied during winding of the innermost layers produce uneven radial growth axially along the reeling drum so that the reel becomes carrot-shaped. This reeling problem is caused partly by superelevation of the web and partly by the carrot shape produced resulting in the nip-pressure profile across the web being greatly elevated at the transition point of the web section.
If the cross-sectional profile of the paper web differs as regards thickness, web tension or elasticity, as in the carrot shape described above, pleating, crushing damage, defects in the web and axial forces in the reel will occur at high nip pressure, which forces tend to displace the reeling axially along the reeling drum if the friction between the paper web and the reeling drum or the layers being applied is insufficient. This displacement may even in the worst case result in a web rupture, which is another reason for striving towards the lowest possible nip pressure. However, with low nip pressure, slipping may occur in the direction of rotation between the surface winding drum and the reeling drum, as well as between the web layers and contrary aims therefore arise concerning the magnitude of the nip pressure. Slipping in the direction of rotation may also occur at web ruptures since these cause the web to become slack as a result of reduced web tension.
The now prevalent use of central driving of the reeling drum, however, reduces the risk of rotational slipping since the reel is then not driven only by the frictional force transmitted through a high nip pressure. However, other problems arising out of the carrot shape remain and it would therefore be a considerable step forward if a well-functioning change of full-width reels could be performed instead which does not give rise to said carrot shape. Changing "full-width" reels in the present context refers to wrapping the entire width of the web around the reeling drum when initiating a new reeling operation. This is to be distinguished from threading using a tapered leader.
When changing reels the web must wrap around the new empty reeling drum, whether reel switching is performed by threading or using full-width transfer. At high web speeds glue is nowadays always applied on the leader, which constitutes the most efficient method hitherto for transferring the continuous web or switching reels. An exemplary device is disclosed in published European Patent Application No. EP 0 765 832 A2.
However, at low web speeds full-width wrapping by means of balloon-blowing is a relatively usual method. Briefly, this entails creating a slack across the full width of the web by somewhat retarding the finished reel. With the aid of compressed air, the fold thus formed is then forced into the nip between the new reeling drum and the surface winding drum, after which the web is cut off. In order to increase the reliability of this type of reel switching, glue or tape is also applied, but only on the actual reeling drum before this comes into contact with the paper web. The reason for this is explained below.
Regardless of the transfer method used for switching reels, it is extremely important that the glue is still adhesive when contact occurs between paper web and reeling drum. It is thus desirable to use the simultaneous and thus more reliable glue spraying for full-width reel switching as well. However, when applying glue by means of spraying, great care must be taken to avoid the glue being misdirected. In earlier attempts at full-width reel switching considerable problems have been encountered with the use of spray pipes across the machine direction because of the hitherto unavoidable and constant dripping of glue from the glue nozzles down onto the paper web below, causing the web layers to adhere to each other and the web to be torn during rewinding. This has prevented installation of glue nozzles across the continuous web and a few nozzles as close to the nip as possible have had to be used. To reduce the risk of glue drops, the glue is often still applied entirely manually, using a brush for instance, that the machine operator must carry with him. Thus, the main reason that automatic nozzles only function satisfactorily when switching reels with the threading method, but not with full-width reel switching, is that no efficient way has yet been found of spreading the glue to the middle of the web other than by manual application.
To enable reel switching at the considerably higher speeds now desired, the method usually used with threading must in some way be replaced by full-width reel switching. Attempts have been made to apply glue with the aid of travelling devices passed across the continuous web during reel switching. See, for example, U.S. Pat. No. 5,213,649. However, this method typically does not permit complete full-width reel switching since the high web speed still results in a slanting web end so that the undesirable carrot shape is obtained.
A key factor for efficient reel switching is thus the type of reel switching used. However, other important factors are control of the nip pressure and quality of the sleeve applied on the reeling drum. If a substandard sleeve, e.g., one with uneven thickness or strength, is pressed against the surface winding drum, the nip will be incompletely closed, sealing only at the places where the sleeve is thickest. The nip pressure must then be increased in order to completely close the nip, but the variations in compressive stress over the transverse profile still remain because of said differences in thickness in the sleeve. These pressure variations are of course greatly aggravated in the helical reeling resulting from threading.
The glue should preferably be applied as an aerosol since poorer result is obtained with a liquid jet. The nozzles are nowadays generally arranged on the primary arms, directed in towards the nip, since the first turns on a new reeling drum generally occur here. A correct nip pressure is most essential during the initial reeling while the reel is still held in the primary arms, since its own weight is then still acting in the direction of the nip. During the secondary reeling, on the other hand, the growing weight of the reel is taken up by the stand rails. Too high a nip pressure will risk crushing the first layers of paper wound on and the nip-pressure control in the primary arms is also extra critical since, because of the leader, the nip width is relatively narrow during the first turns. Glue is sprayed for a brief moment on two occasions, first to facilitate wrapping of the leader around a new empty reeling drum and then, if desired, to attach the opposite end of the web to the finished reel of paper when reel switching has been completed.
Another problem arising during reel switching when either a leader or a string of glue on only a small part of the paper web is used, is that the wrapping fold formed is too long. This wrapping fold occurs since only a small portion of the web width is glued to the new reeling drum, while the rest, constituting the majority, tends to fly off in the machine direction instead of wrapping around said reeling drum. For this reason balloon-blowing with compressed air is usually used to facilitate wrapping, although this unfavorably increases the number of machine parts in a critical area of the reel-up below the continuous paper web. It is thus a distinct end in itself to have as little transverse equipment as possible in the area around the roll nip, i.e., to minimize the number of machine parts both because of the complications arising at a web rupture when the paper web falls down onto said parts and because this facilitates service and repairs, for instance.
Thus, there exists a need in the art for a glue dispensing device which permits full-width changes of reels to eliminate the problems associated with tapered leaders outlined above. Such a device would preferably be operable at high machine speeds, and without risk of excess glue drops falling onto the middle or other portions of the web. Accordingly, full-width reel changes could be accomplished to improve reel quality and the tissue product thus formed.